Fix incorrect variable use in Entity#startRiding

This diff applied and compiled after updating Parchment, but was incorrectly pointing to a field instead of the parameter

fixes #13182

Author: jpenilla

paper-server/patches/sources/net/minecraft/world/entity/Entity.java.patch

@@ -1165,7 +1165,7 @@
          } else if (!entity.couldAcceptPassenger()) {
              return false;
 -        } else if (!this.level().isClientSide() && !entity.type.canSerialize()) {
-+        } else if (!force && !this.level().isClientSide() && !vehicle.type.canSerialize()) { // SPIGOT-7947: Allow force riding all entities
++        } else if (!force && !this.level().isClientSide() && !entity.type.canSerialize()) { // SPIGOT-7947: Allow force riding all entities
              return false;
          } else {
              for (Entity entity1 = entity; entity1.vehicle != null; entity1 = entity1.vehicle) {